1. Field of the Invention
The present invention relates to an electric double layer capacitor (hereinafter referred to as EDLC) which has a large energy density and is capable of rapidly charging or discharging and which is excellent in the charge-discharge cycle resistance.
2. Discussion of Background
Conventional EDLC includes a coin-shaped type wherein an element having a separator sandwiched between a pair of opposing electrodes each having a sheet-shaped polarizable electrode made mainly of activated carbon supported on a current collector, is sealed together with an electrolyte in a metal case comprising a metal cover and a metal casing, by means of an insulating gasket for insulating the cover and the casing, and a winding type wherein an element prepared by winding a pair of opposing sheet-shaped electrodes with a separator interposed therebetween, is accommodated together with an electrolyte in a metal casing and sealed so that the electrolyte will not evaporate from an opening of the casing.
On the other hand, Japanese Unexamined Patent Publications No. 154106/1992, No. 203311/1991 and No. 286108/1992 propose EDLC having an element incorporated which is prepared by laminating a number of electrodes and separators for the purpose of attaining a large capacity with large current. Namely, EDLC is disclosed wherein an element is prepared by laminating many rectangular polarizable electrodes and separators alternately so that separators are disposed between the electrodes, and a positive electrode lead and a negative electrode lead are connected to ends of the respective polarizable electrodes of the element by e.g. crimping. In such a state, the element is accommodated in a casing, an electrolyte is impregnated to the element, and the casing is sealed with a cover.
The electrodes constituting such EDLC are polarizable electrodes composed mainly of activated carbon having a large specific area, for both positive and negative electrodes. Further, in order to obtain a large discharge current, Japanese Unexamined Patent Publication No. 236829/1994 proposes the one in which porous nickel is used for current collectors for both electrodes composed mainly of activated carbon.
Japanese Unexamined Patent Publication No. 14882/1989 proposes a secondary cell wherein an electrode composed mainly of activated carbon is used as the positive electrode, and a composite having lithium ions occluded in a molded product of carbon material having a lattice spacing d.sub.002 as measured by X-ray ray diffraction (hereinafter referred to as lattice spacing d.sub.002) of from 0.338 to 0.356 nm, is used as the negative electrode.
With conventional EDLC wherein polarizable electrodes composed mainly of activated carbon are used for both electrodes, the working voltage per unit element is about 1.0 V in the case of EDLC with an aqueous electrolyte or about 2.5 V in the case of EDLC with a nonaqueous electrolyte, although it depends also on selection of the solvent and the solute to be combined. In order to obtain a larger quantity of electric energy (in order to obtain a higher energy density), EDLC having a higher working voltage is desired.
Further, EDLC or a cell wherein a polarizable electrode composed mainly of activated carbon is used as the positive electrode and a carbonaceous material having lithium or lithium ions occluded, is used for the negative electrode, is not suitable for rapid charging and discharging, since its internal resistance is substantial, and thus has a drawback that it lacks in charge-discharge cycle durability.
In order to increase the capacity of EDLC, an attempt to increase the capacity by means of activated carbon having a large specific surface area, has been made. However, the specific surface area of activated carbon is limited to about 3000 m.sup.2 /g at the largest, and even if activated carbon having a larger specific surface area, is employed, the energy density can not be improved, since the pore volume of activated carbon is large. Thus, the capacity per unit weight of EDLC wherein activated carbon having a large specific surface area is employed, is restricted. However, in order to secure a longer backup type, a larger capacity is desired.
At present, coin-shaped EDLC of small size is frequently used for memory backup. Whereas, IC used to be operated at 5 V, and accordingly a working voltage exceeding 5 V used to be obtained by connecting two or more EDLCs in series. However, recently, IC can be operated at 3 V, and the memory backup can be conducted at 3 V. Accordingly, it has been desired to develop EDLC which has a working voltage exceeding 3 V with a single EDLC.
Further, EDLC capable of charging and discharging with a large current of at least 10 A is expected to be useful as a power source for an electric car or for temporary storage of its regenerative braking energy. Accordingly, it has been desired to develop EDLC which has a sufficiently high energy density and which is capable of rapid charging and discharging and excellent in the charge-discharge cycle durability.